My invention relates to automatic control valves operated either by a modulating electrical or pneumatical signal to control very minute flow rates, as is customary in the operation of pilot plants in the chemical or petroleum industry (see U.S. Pat. No. 3,997,141). Control valves presently in use for this purpose are either of the low-lift variety utilizing a needle-type plug or the long-lift cylindrical plug type with precision machined longitudinal scratch, commonly referred to as micro-splined plugs. Both of these types exhibit some problems. The needle-type plug is commonly limited to a tapered angle of 30.degree. to avoid self-locking against the seat ring bore. This in turn reduces the stroke whenever small orifice sizes are required. For example, the maximum usable stroke for control with a 1/32 inch orifice is only 0.055 inch, i.e., much too short to achieve any meaningful positioning accuracy with conventional actuating devices. Another drawback is the very high tendency for fluids to cavitate with needle plugs resulting in early destruction of the valve parts.
The micro-splined plugs require extremely accurate machining of splined grooves having depths of less than 0.001 inch. The minimum controllable area, limiting the lowest flow rate that can be regulated, is given by the radial clearance between the external diameter of the plug and the orifice bore. With a normal radial clearance of 0.00025 inch the minimum controllable area of such a plug with 1/4 inch diameter is 2.times.10.sup.-4 in.sup.2 or 25% of the maximum area of a 1/32 inch dia. orifice, thereby limiting the ratio of max. to min. control area or rangeability to less than 4:1.
My invention, on the other hand, eliminates the amount of minimum controllable area through the use of an elastomeric or plastic sleeve.
A further objective of my invention is the provision of a valve trim with tight closure capabilities. My invention accomplishes this goal by having the fluid assist the circumferential sealing capabilities of said plastic sleeve.
Finally, my invention provides for a throttling valve trim that can be machined very simply in order to get a very precise and repeatable flow characteristic without resorting to unusual precision manufacturing processes.
These and other objectives of my invention will become more easily understood from the following detailed description: